The invention relates to a fuel injection device for an air-compression and self-ignition internal combustion engine having a high-pressure injection pump in which an injection line leads from each pump element thereof to a line bifurcation which is connected on the one hand to a main injection nozzle and on the other hand to a preinjection nozzle. The preinjection nozzle comprises a nozzle needle guided in a nozzle body which lifts off from its valve seat against the force of a restoring spring in response to fuel pressure being led to the preinjection nozzle and in a direction counter to the direction of flow of the fuel in a feed bore of the preinjection nozzle.
German Unexamined Pat. Application No. 33 30 987 discloses a fuel injection device which comprises a high-pressure injection pump and an auxiliary preinjection pump as an extraneous control for the preinjection nozzle.
Starting from the high-pressure injection pump, a high pressure injection line leads in a conventional manner to the main injection nozzle. A low pressure feed pump conveys fuel out of a tank into a delivery line at a low pressure, via a line bifurcation and an interposed damping reservoir, into a pump working space of the high pressure injection pump and then via an onward-leading line and a non-return valve, into a working space of the hydraulic auxiliary pump for the preinjection. The line systems for preinjection and main injection are separate.
The object on which the invention is based is to simplify the fuel injection device having preinjection and main injection and furthermore, to make possible a sharp division between the end of preinjection and the start of main injection.
This object is obtained by having a preinjection nozzle include a stepped piston in its nozzle body, which is pressed onto a sealing seat by spring force and is longitudinally displaceable in the body in response to fuel in the feed bore leading from the fuel injection pump to a connecting channel and wherein a closure initiating fuel pressure, which is above an opening fuel pressure of the nozzle needle, moves the needle nozzle into the position for ending preinjection of fuel.
Accordingly, the invention results in an automatically controlled system which operates without extraneous influence, such as an auxiliary pump, and which by virtue of the special nature of the nozzle needle closing function of the preinjection nozzle, prevents after dribble and ensures a sharp division between preinjection and main injection. This produces positive effects with regard to combustion noises, fuel consumption and exhaust gases by virtue of shortening of the duration of the preinjection phase as a consequence of the sharply falling end of preinjection, while at the same time still maintaining the same preinjection quantity as other systems.
It is advantageous if the stepped piston is arranged coaxially to the nozzle needle and is provided with a closing pin which cooperates to close the nozzle needle. The stepped piston has two piston parts of different diameters, of which the larger piston part is guided in the guiding body of preinjection nozzle and the smaller piston part, together with the guiding body, forms an annular chamber which can be connected to a connecting channel leading to the bifurcated fuel line when the closure initiating pressure is reached.
The stepped piston should have the sum of the areas of the annular surface formed between the piston parts and of the front face of the smaller piston part that faces the area of the connecting channel side be greater than the pressure shoulder of the nozzle needle to which fuel pressure is applied for causing opening of the nozzle needle.
It is also advantageous if the restoring spring, intended for biasing the nozzle needle, also simultaneously forms the closing spring for the stepped piston and surrounds the closing pin driving the nozzle needle. The spring is supported on one side by a nozzle needle spring plate and on the other side by a stepped piston spring plate. Also, when the nozzle needle is resting on its opening stop, the stepped piston is separated from the nozzle needle by an interval permitting a defined inactive stroke of the piston prior to the time its closing pin operates to close the needle nozzle of the preinjection nozzle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.